U.S. Pat. No. 4,196,282 describes a process in which a solution of cellulose in N-methylmorpholine-N-oxide (hereinafter NMMO) and water is formed. Cellulose solutions of this type can be used to produce cellulose fibers or other shaped bodies of a cellulose base. For this purpose, the cellulose is extruded by spinning nozzles, e.g. a spinneret, into a precipitating or coagulating bath. The use of a mixture of NMMO and water as solvent has a number of advantages. For example, it allows operation with a closed solvent cycle since NMMO can be recovered and reused both in dissolving the cellulose for preparing the spinning solution and in the coagulating bath.
In the NMMO process, the cellulose dissolved in NMMO and water is coagulated in an NMMO containing coagulating bath, the fibers are then washed and the washing water recycled to the precipitating bath. In regenerating of the precipitating bath it is evaporated to allow recovery of the NMMO concentrate which can be used to form fresh solutions of the cellulose while the distillate can be employed for washing the fibers.
In prior art systems of this type, the NMMO concentration in the precipitating bath has been limited to about 20 to 25% since higher concentrations appeared to have a detrimental effect on the characteristics of the fibers. It is, of course, desirable to raise the concentration of the NMMO in the precipitating or coagulating bath so that smaller quantities of water need to be evaporated to regenerate this bath.